1. Field of the Invention
The present invention relates to position control and more particularly to the control of aircraft utilizing signals from Global Position Systems (GPS) and Differential Global Position Systems (DGPS).
2. Description of the Prior Art
GPS and DGPS systems are well known in the art. GPS systems utilize a number of orbiting position transmitting satellite stations and a receiver on the aircraft to determine the position of the aircraft with respect to ground. With the received information from the satellites, the receiver can determine a number of aircraft parameters such as position, speed and even attitude. DGPS systems utilize the basic GPS system and the orbiting position transmitting satellite stations but also uses one or more ground receiver systems to greatly improve the accuracy of the GPS information to the aircraft. Because the ground station position can be precisely known, any errors which may occur in the transmission from the orbiting satellites can be checked and a signal sent to the aircraft indicating the errors so that the airborne receiver can modify the signals it receives from the satellites and determine the aircraft parameters to a high degree of precision. GPS without ground station modification are used primarily for aircraft travel between airfields while DGPS systems are used primarily for travel around airfields, for precision landings and for missed approaches etc.
There is always concern about the amount of confidence that can be attributed to a GPS or DGPS position determination and accordingly systems have been developed to provide information as to the integrity of the signals. In a system identified as RAIM (Receiver Autonomous Integrity Monitor), the integrity of a GPS signal has been heretofore determined. For example, using RAIM, it can be calculated that the position signal from the airborne receiver has a certain high percentage, (say 99.9%) chance of being within a certain distance (say one third nautical mile or about 2,025 feet) of the actual position. This is satisfactory for aircraft flying between airfields, but not accurate enough for near airfield positions, particularly during precision landings. Federal guidelines have been established for aircraft of different sizes and different altitudes which create a sort of "tunnel" that an aircraft must be within and with an integrity of 99.9% for long distance travel and with an integrity of 1 part in 10.sup.-7 or of about 99.9999999 for precision landings. For example, Federal guidelines may require an aircraft in a precision landing mode at 200 feet altitude, to be within 110 vertical feet era desired trajectory and within 425 horizontal feet of the desired trajectory with an integrity of 99.9999999%.
Honeywell has developed a system with sufficient accuracy and integrity to enable automatic precision landings in most instances. This system is described and claimed in an application of Mats Brenner entitled Differential Satellite Positioning System Ground Station with Integrity Monitoring filed Jun. 30, 1993 Ser. No. 08/497,895 and assigned to the assignee of the present invention. In the Brenner system, the position of an aircraft as determined by the DGPS, may have the high 99.9999999% integrity for a much smaller distance (say 30 feet) than has heretofore been possible. This allows aircraft to be controlled mound airfields and in precision landing modes with sufficient accuracy to meet the Federal guidelines.
In either the RAIM or the Brenner DGPS system, a possible problem is encountered. Assume, that an aircraft is moving along a prescribed path and the position is determined to be within X number of feet of the actual position with a certain confidence level and a malfunction occurs. If the malfunction is in the aircraft controls (such as the autopilot or servos of the aircraft or the flight management system) the aircraft may start flying off of the prescribed course. However, when its position becomes outside the tunnel determined by the Federal guidelines, the GPS or DGPS can be programmed to produce a signal which will cause an alarm to alert the pilot to the malfunction and to permit him to disengage the auto pilot to move back into the tunnel. However, if a malfunction occurs in the GPS or the DGPS, the aircraft could fly off the prescribed course and the GPS or DGPS would not detect it in which case no alarm would be given. This would be particularly hazardous during precision landings.
Standard fail safe systems could be employed which would add a second parallel GPS or DGPS and compare the outputs of the two parallel systems so that if they differed, then a malfunction would be presumed and an alarm would be given. Such fail safe systems require not only duplication of the measuring apparatus but also the use of a comparator which introduces the problem that the comparator itself may malfunction. Triple redundant systems can reduce this problem to a very low level of probability but further duplicate measuring systems are needed and the result is exceedingly costly in equipment, weight and space.